1. Technical Field
The present invention relates to transducers that generate acoustic energy in the frequency range around one megahertz and more particularly to a system that delivers a uniform amount of acoustic energy to the surface of a rotating object.
2. Background Information
It is well-known that sound waves in the frequency range of 0.4 to 2.0 megahertz (MHz) can be transmitted through liquids and used to clean particulate matter from damage sensitive substrates. Since this frequency range is predominantly near the megahertz range, the cleaning process is commonly referred to as megasonic cleaning. Among the items that can be cleaned in this manner are semiconductor wafers in various stages of the semiconductor device manufacturing process, disk drive media, including compact disks and optical disks, flat panel displays and other sensitive substrates.
Megasonic acoustic energy is generally created by exciting a crystal with radio frequency AC voltage. The acoustic energy generated by the crystal is coupled through an energy transmitting member (a resonator) and into a fluid. Frequently, the energy transmitting member is a wall of the vessel that holds the fluid, and a plurality of objects are placed in the vessel for cleaning. For example, U.S. Pat. No. 5,355,048, discloses a megasonic transducer comprised of a piezoelectric crystal attached to a quartz window (resonator) by several attachment layers. The megasonic transducer operates at approximately 850 KHz. Similarly, U.S. Pat. No. 4,804,007 discloses a megasonic transducer in which energy transmitting members comprised of quartz, sapphire, boron nitride, stainless steel or tantalum are glued to a piezoelectric crystal using epoxy.
It is also known that megasonic cleaning systems can be used to clean single objects, such as individual semiconductor wafers. For example, U.S. Pat. No. 6,021,785 discloses the use of a small ultrasonic transmitter positioned horizontally adjacent to the surface of a rotating wafer. A stream of water is ejected onto the surface of the wafer and used to both couple the acoustic energy to the surface of the disk for sonic cleaning and to carry away dislodged particles. Similarly, U.S. Pat. No. 6,039,059 discloses the use of a solid cylindrically-shaped probe that is placed close to a surface of a wafer while cleaning fluid is sprayed onto the wafer and megasonic energy is used to excite the probe. In another example, U.S. Pat. No. 6,021,789 discloses a single wafer cleaning system that uses a plurality of transducers arranged in a line. A liquid is applied to a surface of the wafer and the transducers are operated so as to produce a progressive megasonic wave that carries dislodged particles out to the edge of the wafer.
Briefly, the present invention is a transducer that delivers an approximately uniform amount of acoustic energy to every point on the surface of a rotating object. The transducer comprises a piezoelectric crystal attached to a resonator. Electrically conductive layers on both sides of the crystal are used to create an electric field which drives the crystal. Preferably, the transducer generates acoustic energy in the frequency range of 0.4 to 2.0 MHz.
In one embodiment, the crystal in the transducer is wedge shaped so that the active acoustic surface area of the crystal increases as the radius of the rotated object increases. This means that the amount of acoustic energy delivered to the object increases with increasing radius. However, since the time that a region of the object spends under the transducer varies inversely with the radius, the total amount of acoustic energy delivered to each unit of surface area on the surface of the object is the same. This is useful in situations where the acoustic energy is used to assist some type of chemical reaction (e.g. sonochemistry) occurring on the surface of the object, and it is desired to have the chemical reaction proceed uniformly over the whole surface. It is also useful where uniform acoustic cleaning of the object is desired, as well as in other situations where uniform exposure to the megasonic acoustic energy is desired.
In another embodiment, the crystal has a rectangular shape, but the electrically conductive layers on both sides of the crystal are given the wedge shape. This causes the crystal to deliver an amount of acoustic energy to the object that increases with increasing radius, just as if the crystal itself had the wedge shape.